1. Field of the Invention
The present invention relates to a fluid torque converter for an automotive transmission system, and in particular to a novel stator assembly for such a fluid torque converter.
2. Background of the Invention
According to a conventional stator assembly of a fluid torque converter for an automotive transmission system, stator blades are integrally cast with an annular inner shell and a core ring to the end of simplifying the manufacturing process and reducing the manufacturing cost. An example of such a stator assembly is disclosed in Japanese utility model laid open publication (kokai) No. 3-35359. Typically, the die casting assembly consists of two halves which can be drawn apart or separated apart in the axial direction when removing the cast product. To make it possible, the adjacent blades do not overlap each other as seen from the axial direction, and the profile of the surfaces of the core ring and the inner shell facing the flow passage are linear in the axial direction. Because the stator blades have a certain attack angle with respect to the axial direction, the circumferential surface of the core ring or the inner shell is required not to interfere with the parts of the die assembly which form the profile of the stator blades. If the core ring or the inner shell is contoured, for instance with a lens-shaped cross section, it is impossible to draw apart the two halves of the die assembly in the axial direction.
In a fluid torque converter of this type, particularly in a stall condition or when the speed ratio (turbine rpm/pump rpm) e=0, the flow velocity of the circulating fluid is relatively high, and the inflow angle with respect to the stator blade (refer to numeral 21 of FIG. 6) is relatively large as indicated by arrow B in FIG. 6. Further, when the three dimensional flow pattern of the circulating fluid is considered, it curves around the core ring 22 as indicated by the imaginary line in FIG. 7. The inflow angle progressively diminishes with the increase in the speed ratio e, and the fluid flow eventually takes place in the direction indicated by dotted arrow C in FIG. 6. With further increase in the speed ratio e, the fluid flow is ultimately directed toward the rear surface of the stator blade 21 (which is opposite to the pressure surface of the stator blade 21 upon which the fluid flow normally impinges), and the torque amplifying property of the torque converter is lost.
Sometimes, a relatively thick airfoil profile is used for the stator blade, instead of a relatively thin blade profile which is characterized by a relatively uniform thickness over an entire base line thereof and commonly used for the pump and the turbine, so that the fluid loss may be reduced over such a wide range of the inflow angle. However, because the inner circumferential surface of the core ring 22 is not smoothly curved along the axial direction as opposed to the pump and the turbine, the axial fluid flow curving around the core ring 22 tends to be separated from the inner circumferential surface of the core ring 22 as indicated by arrows in FIG. 8, and this substantially increases the fluid loss. As a result, the torque ratio tends to drop sharply near the stall point involving a relatively high fluid flow velocity.